1. Field of the Invention
The present invention relates in general to a semiconductor device and its manufacturing process, and in particular to a thin film transistor used in the semiconductor device and its manufacturing process, and a display device and its manufacturing process.
2. Description of the Related Art
Active matrix type liquid crystal displays (LCD's) utilizing thin film transistors have recently been recognized as high quality display devices. Dot matrix type LCD's, using dots arranged in the form of a matrix, are classified into two types: active and simple matrix systems.
In simple matrix systems, the liquid crystals of pixels arranged in the matrix are directly driven in synchronism, with scanning signals. The pixel portion of liquid crystal panel of an LCD is formed by using electrodes and liquid crystals. Accordingly, when the number of scanning lines is increased, the driving time (i.e., duty cycle) for each pixel is shortened. This results in the reduction of contrast.
In active matrix systems, each of pixels arranged in the matrix has an active element as a pixel driving element and a signal holding element (i.e., pixel capacitance). Both the driving and holding elements are integrated in the pixel. Each pixel performs signal storage operation, so that liquid crystals are semi-statically driven. In other words, each pixel driving element functions as a switching element controlled in response to a scanning signal. When a pixel driving element is turned on, the associated pixel receives a data signal indicative of display data via the pixel driving element. This drives the liquid crystal. Thereafter, when the pixel driving element is turned off, the data signal applied to the pixel is stored as an electric charge in an associated signal holding element. Based on the accumulated charge, the drive control of the liquid crystal is maintained until the pixel driving element is turned on again. Therefore even with reduced driving times for each pixel, due to increased numbers of scanning lines, the drive control of liquid crystal will remain unaffected and no reduction in contrast will occur. Thus, active matrix systems provide displays having a much higher quality than that of simple matrix systems.
Active matrix systems are generally classified by the type of pixel driving elements used: a three terminal type transistor. The transistor type matrix system, although difficult to manufacture compared with the diode type matrix system, lends itself to improved contrast and resolution characteristics. Transistor type matrix systems moreover, allow the possibility to achieve CRT quality in a LCD device.
Thin film transistors (TFT's) are generally used as pixel driving elements in transistor type LCD's. An active layer in a TFT is a semiconductor thin film formed on an insulation substrate, and is generally an amorphous silicon or a polycrystalline silicon film. A TFT using an amorphous silicon film as an active layer is referred to as an amorphous silicon TFT, while a TFT using a polycrystalline silicon film as an active layer is referred to as a polycrystalline silicon TFT. Polycrystalline silicon TFT's have an advantage in that they have higher field effect mobility and hence higher drivability compared with amorphous silicon TFT's. Therefore, polycrystalline silicon TFT's can be used not only as pixel driving elements but also as elements forming logic circuits. Thus, the use of polycrystalline silicon TFT's allows the pixel portion of a LCD and the peripheral driving circuits to be integrated on the same substrate. This makes it possible to form polycrystalline silicon TFT's as pixel driving elements and polycrystalline silicon TFT's as peripheral driving circuits during the same manufacturing step.
FIG. 1 is a block diagram showing a configuration of a general active matrix type LCD. A pixel portion 101 has a plurality qf scanning lines (i.e., gate lines) G.sub.1, . . . , G.sub.n, G.sub.n+1, . . . , G.sub.m and a plurality of data lines (i.e., drain lines) D.sub.1, . . . , D.sub.n, D.sub.n+1, . . . , D.sub.m. The gate and drain lines extend perpendicularly to each other, and the pixels 102 are provided at the intersections thereof. The gate lines are connected to a gate driver 103 which supplies the gate lines with gate signals i.e., scanning signals. The drain lines are connected to a drain driver 104 which supplies the drain lines with data signals i.e., video signals. These drivers 103 and 104 form a peripheral driving circuit 105. An LCD, having at least one of the drivers 103 or 104 formed on the substrate on which the pixel portion 101 resides, is generally referred to as an integral driver type LCD or driver-incorporated type LCD. The gate driver 103 may be provided on both sides of the pixel portion 101. Further, the drain driver 104 may be provided on both sides of the pixel portion 101.
FIG. 2 is an equivalent circuit for a pixel 102 provided between a gate line G.sub.n and a drain line D.sub.n. The pixel 102 is formed by a TFT 110 as an active element, a liquid crystal cell LC, and an auxiliary capacitance CS. The TFT 110 has a gate connected to the gate line G.sub.n and a drain connected to the drain line D.sub.n. The source of the TFT 103 is connected to the auxiliary capacitance CS and a display electrode (i.e., pixel electrode) LCE1 of the liquid crystal cell LC. The liquid crystal cell LC and auxiliary capacitance CS form the above-described signal holding element. A voltage V.sub.com is applied to a common electrode LCE2 of the liquid crystal cell LC. The common electrode LCE2 of each liquid crystal cell LC is an electrode shared by all the pixels 102. An electrostatic potential is established between the display electrode LCE1 and common electrode LCE2. The auxiliary capacitance CS has a first electrode CSE1 connected to the source of the TFT 110 and a second electrode CSE2 to which a constant voltage V.sub.R is applied. The second electrode CSE2 may be connected to the adjacent gate line G.sub.n+1.
In the pixel portion 101, data signals must be held in the signal holding elements (i.e., LC and CS). during one frame, i.e., during the period from when the pixel driving elements are turned off to when the elements are turned on. Accordingly, each of the polycrystalline silicon TFT's 110 as pixel driving elements experience a small leakage current when turned off. However, it is desirable to have a peripheral driving circuit 105 that operates at high speeds. Consequently, the polycrystalline silicon TFT's forming the peripheral driving circuit 105, when turned on, must be supplied with large currents.
The lower the field effect mobility in an active layer of a polycrystalline silicon TFT is, the smaller the turned-off current of the TFT is. The higher the field effect mobility in the active layer is, the larger the current of the conducting TFT. Therefore, the active layers of the polycrystalline silicon TFT's, functioning as the pixel driving elements at the pixel portion 101, preferably should have low field effect mobility. On the other hand, the active layers of the polycrystalline silicon TFT's, forming the peripheral driving circuit 105, preferably should have high field effect mobility.
In order to satisfy both of these performances conditions for the pixel portion 101 and the peripheral driving circuit 105 simultaneously, it is necessary to optimize the field effect mobility of the polycrystalline silicon films used for the portions 101 and 105. It is therefore desirable to have a method or process of manufacturing a polycrystalline silicon film whose field effect mobility can be adjusted as needed.
In LCD's which are currently available, reliably holding data signals in the signal holding elements for the period of one frame is more important than improving the operating speed of the peripheral driving circuit 105. In conventional integral driver type LCD's, therefore, the field effect mobility of polycrystalline silicon films, used for both of a pixel portion 101 and a driving circuit 105, is set low in an attempt to make the turned-off current of polycrystalline silicon TFT's small. This reduces the turned-on current of polycrystalline silicon TFT's forming a peripheral driving circuit 105, and consequently reduces the operating speed of the driving circuit 105. At this point in time, achieving optimum levels of pixel performance for pixels at portion 101 and at peripheral driving circuit portion 105 has not been successful.